Radiation detectors provided with a specially adapted external filter system, such as for example interference filters or monochromators, are often used to detect radiation with a predefined spectral sensitivity distribution that has a maximum at a predefined wavelength λ0. Such detectors are distinguished by very good matching of the predefined spectral sensitivity distribution, but are usually comparatively labor- and cost-intensive to handle and produce. Furthermore, they often have high spatial requirements, precluding or limiting their ability to be used in small spaces.
If the predefined spectral sensitivity distribution is that of the human eye, then a silicon photodiode is often used to detect incident radiation according to that sensitivity.
The sensitivity of a photodiode depends on—among other things—the wavelengths of the incident radiation. At wavelengths in excess of a boundary wavelength corresponding to the band gap, the sensitivity is very low, since for incident radiation in this wavelength range the band gap of the functional material in the active region of the diode—for example Si—is greater than the energy of the incident radiation and is therefore insufficient to generate electron-hole pairs. By the same token, sensitivity declines in the range of diminishing wavelength, since as the wavelength decreases, the electron-hole pairs that are produced, for example by surface recombination, progressively cease to contribute to the photocurrent. In the intermediate range, the sensitivity of the diode has a maximum which in a conventional silicon photodiode can occur at above 800 nm.
The use of such a silicon photodiode as a detector having the spectral sensitivity distribution of the bright-adapted human eye, which shows a maximum sensitivity at about 555 nm, requires additional expenditure, since the wavelengths of the sensitivity maxima sharply differ from each other and the two spectral sensitivity distributions therefore match relatively poorly. The matching of detector sensitivity to the sensitivity distribution of the human eye can be improved through the use of additional filters. The additive result is a sensitivity distribution close to that of the human eye.